1. Field of the Invention
The invention concerns satellite navigation systems (of RNSS (Radio Navigation Satellite System) or GNSS (Global Navigation Satellite System) type), and more precisely the generation of integrity messages that are transmitted to their navigation receivers, in addition to navigation messages.
Here “satellite navigation system” means any system dedicated to navigation and including a constellation of satellites placed in orbit around a heavenly body (for example the Earth), a set of (terrestrial or spatial) satellite surveillance stations, and a computer center. It could be one of the existing systems (GPS, EGNOS, WAAS), for example, or one of the planned systems (GALILEO, COMPASS), or one of their equivalents or derivatives.
Moreover, here “navigation message” means a message that is broadcast to navigation receivers of users of a satellite navigation system and includes navigation information relating to its orbital position and/or its synchronization (offset of its internal clock relative to the reference clock of the system).
Furthermore, hereinafter “integrity information” means data representative of the reliability values of information on the position and/or the offsets of the clocks of the satellites, used to produce user position and/or clock information, and contained in a navigation message.
2. Description of the Prior Art
As the person skilled in the art knows, the quality of the surveillance of the integrity of the users of the navigation receivers has an impact on the confidence that a user can have in the position that his (navigation) receiver has determined. That surveillance quality depends essentially on the integrity surveillance conditions, i.e. the combination of the number of active (i.e. nominal or degraded) surveillance stations therefore capable of surveillance of each satellite, errors in pseudo-distance measurements effected by those surveillance stations, the geometrical distribution (positions) of the surveillance stations vis à vis the satellite(s), and the algorithms used to effect this surveillance. Failure of at least one surveillance station or a transmission problem will therefore degrade integrity surveillance performance and it is crucial to inform users of them as quickly as possible.
In current navigation systems (with integrity), an integrity surveillance quality indicator (ISQI) is broadcast in an integrity message sent to the navigation receivers for each satellite under surveillance by the integrity surveillance “segment” of the system. In the case of the EGNOS or WAAS system, for example, the ISQI is called the User Differential Range Error (UDRE), and in the case of the GALILEO system the ISQI is called the Signal In Space Monitoring Accuracy (SISMA).
It being beneficial for a navigation receiver to use as many satellites as possible to enhance the estimate of its position, it is therefore important for it to have the greatest possible number of ISQI. Now, not only is the number of navigation satellite constellations increasing, but also the number of satellites per constellation is tending to increase (the standard GPS constellation is defined for 24 satellites but in fact comprises 27 or 28 satellites, and GALILEO could include up to 36 satellites). Consequently, the standard integrity messages are going to tend to comprise an ever higher number of ISQI. This trend runs the risk of becoming further accentuated if the same “network” of surveillance stations is used for several different constellations of navigation satellites (for example, an integrity surveillance segment common to the GPS, GLONASS and GALILEO constellations would have to manage more than 90 satellites.).
In the case of failure of a portion of the integrity surveillance segment, it is necessary to inform users of the modifications of the integrity surveillance capacity with respect to several tens of satellites, by sending as many ISQI as there are satellites concerned. This gives rise to a real problem because of the limited bandwidth dedicated to the navigation signals (for example, the set of SBAS (Satellite Based Augmentation System) type messages can manage only 51 satellites in a static manner, the set of messages planned for the GALILEO system can manage only 36 satellites in a static manner, and dynamic management, necessitating identification of each satellite (7 bits for 90 satellites), offers even lower capacity). The mechanism for alerting users by way of the standard integrity messages therefore proves to be poorly adapted or even not adapted at all to situations in which the number of satellites under surveillance by the integrity surveillance segment is high, typically greater than fifty.
An object of the invention is therefore to improve upon this situation.